(b) That's probably wrong. In this example, the steps A took to mitigate the loss have ended up increasing it. He spent more money acquiring replacement seeds, and got even less for the tomatoes in the long run. But as long as the plaintiff has acted reasonably in the circumstances at the time of the breach in taking the action they did, then even if the result is counter-productive, the plaintiff is entitled to recover both the actual loss suffered, as well as the additional expenses incurred.
Although, with the wisdom of hindsight, B might argue that A should not have taken the risk of planting a late crop, A's action does not seem unreasonable in the circumstances at the time of the breach. He would therefore likely succeed in claiming both the extra cost of the replacement seed and the loss of profit due to the delay in marketing the crop.